Generally, vacuum cleaners generate a suction force that draws in dust together with ambient air, and then separates and collects the dust from the air using a dust collecting device. FIG. 1 shows an example of a prior art vacuum cleaner. Referring to FIG. 1, a vacuum cleaner 1 comprises a suction brush 2, an extension pipe assembly 3, and a cleaner body 4. The suction brush 2 has a suction port (not shown) at a lower side to draw in dust from a surface being cleaned. The extension pipe assembly 3 interconnects between the suction brush 2 and the cleaner body 4 and forms a passage for the dust drawn in through the suction brush 2. The cleaner body 4 includes a dust collecting device 6 and a fan assembly 7. The dust collecting device 6 separates and collects the dust from the drawn-in air. A dust bag or a cyclone dust collector can be used for the dust collecting device 6. The fan assembly 7 generates a suction force for drawing in the air.
The fan assembly 7 comprises a motor 9, an impeller (not shown), and a diffuser 8. The impeller is connected to a rotary shaft of the motor 9 and rotated by the motor 9, thereby generating the suction force for drawing in the air. The diffuser 8 induces the air being discharged from the impeller toward the motor 9. Therefore, the drawn-in air cools the motor 9 and exits to the outside passing through a discharge port 5 of the cleaner body 4.
The conventional fan motor, as described above, generates a wind noise due to a flow field formed around the air suction port that collides with wings of the impeller, and generates a blade passing frequency (BPF) noise caused by interaction between the rotating impeller wings and the fixed wings of the diffuser. The BPF noise refers to a peak noise generated at a BPF which the impeller having certain number of wings passes through, and a frequency corresponding to an integral multiple of the BPF. The BPF noise in a vacuum cleaner is often very offensive to a user because it is a strong high-frequency sound.
Korean Patent Registration No. 457551 discloses a fan motor for solving such a problem, in which an upper end of the impeller is protruded more than a lower end and a angled part is formed so that a lower end of a diffuser entrance is protruded more than an upper end. In this structure, the air passes through the lower end of an impeller entrance and is introduced toward the diffuser first, thereby preventing an air whirlpool from being formed at the upper end of the diffuser entrance and accordingly reducing the BPF noise.
However, because the lower end of the diffuser entrance protrudes, the airflow may become turbulent at the lower end of the diffuser entrance, accordingly causing multiple air whirlpools. Simultaneously, the diffuser wings, leading ends of which are angleded at the same angle, may cause BPF noise because the same frequencies are superposed.